1. Field of the Invention
The present invention relates to an engaging structure of two cylindrical members, at least one of which is rotatable relative to the other about a common axis.
2. Description of the Related Art
In the case where two cylindrical members (e.g., two cylindrical members provided in a zoom lens barrel of a zoom lens camera) need to be axially engaged so that at least one of the two cylindrical members is rotatable relative to the other cylindrical member about a common axis, it is often the case that a plurality of engaging projections are formed on one of the two cylindrical members (i.e., first cylindrical member) while a circumferential groove in which the plurality of projections can be slidably fitted is formed on the other cylindrical member (i.e., second cylindrical member). The circumferential groove is generally formed between a couple of front and rear flanges each extending out radially from the outer peripheral surface of the second cylindrical member. One of the flanges is provided with a plurality of cut-away portions through which the plurality of engaging projections can enter the circumferential groove in the axial direction. After the plurality of engaging projections are inserted into the circumferential groove through the plurality of cut-away portions, the first and second cylindrical members are engaged with each other by means of rotating the first and second cylindrical members relative to each other about a common axis by a certain amount of rotation (angle of rotation). In this state of engagement, the plurality of projections of the first cylindrical member are prevented from disengaging from the circumferential groove of the second cylindrical member in the axial direction. Due to the nature of such an engaging structure, the maximum relative amount of rotation (angle of rotation) between the first and second cylindrical members formerly depends upon the number of the engaging projections of the first cylindrical members. For instance, if only one engaging projection is formed on the first cylindrical member while a corresponding cut-away portion is formed on the second cylindrical member, the maximum relative angle of rotation can be set close to 360 degrees; which is, however, impractical because stable engagement cannot be ensured between the two cylindrical members. In the case where three engaging projections are formed on the first cylindrical members circumferentially at regular intervals while corresponding three cut-away portions are formed on the second cylindrical member, the maximum relative angle of rotation is approximately 120 degrees. In order to ensure stable engagement between the two cylindrical members, it is preferable that three or more engaging projections are formed on the first cylindrical members while the corresponding number of cut-away portions are formed on the second cylindrical member. However, as the number of the engaging projections increases, the maximum relative angle of rotation decreases because the number of the cut-away portions increases.
In such a conventional engaging structure using the plurality of engaging projections and the circumferential groove, in the case where it is necessary to make the maximum relative angle of rotation larger than a usual case, e.g., in the case of making the maximum relative angle of rotation over 120 degrees even though the first cylindrical member is provided with three engaging projections circumferentially at regular intervals, it is possible to make the maximum relative angle of rotation over 120 degrees by proving an extra rotatable member (relaying member) between the first and second cylindrical members that is rotatable relative to each of the first and second cylindrical members, or by simply decreasing the number of the engaging projections. Providing such an extra rotatable member is costly, space-consuming; and thus, not preferable. On the other hand, decreasing the number of the engaging projections generally causes unstable engagement between the two cylindrical members, which is not preferable either.
A different type of conventional engaging structure using a plurality of engaging projections formed on the first cylindrical member and a circumferential groove formed on the second cylindrical member is known. In this conventional type, each engaging projection of the first cylindrical member is made of a resilient material and is formed to be slightly greater than the corresponding cut-away portion formed on the second cylindrical member, so that each engaging projection is inserted into the circumferential groove through the corresponding cut-away portion by being deformed to pass through the same. In this type, it is possible to make the maximum relative angle of rotation between the two cylindrical members large, regardless of the number of the engaging projections. However, since the engaging projections need to be hard-pressed against the cut-away portions to pass through the same so as to be fitted in the circumferential groove, it is not so easy for the two cylindrical members to be engaged and assembled. Furthermore, in this type, there is a high probability of the engaging projections disengaging from the circumferential groove as the cut-away portions cannot be formed much smaller than the engaging projections.